Various bases are considered for the classification of timbers. The following are the important basis:
(i) Mode of growth
(ii) Modulus of elasticity
(iii) Durability
(iv) Grading
(v) Availability.
(i) Classification Based on Mode of Growth: On the basis of mode of growth trees are classified as
(a) Exogeneous and
(b) Endogeneous
(a) Exogeneous Trees:
These trees grow outward by adding distinct consecutive ring every year. These rings are known as annual rings. Hence it is possible to find the age of timber by counting these annual rings. These trees may be further divided into
(1) Coniferous
(2) deciduous.
Coniferous trees are having cone shaped leaves and fruits. The leaves do not fall till new ones are grown. They yield softwood.Deciduous trees are having broad leaves. These leaves fall in autumn and new ones appear in springs. They yield strong wood and hence they are commonly used in building construction.The classification as softwood and hardwood have commercial importance.
The difference between softwood and hardwood is given below:
1. In soft wood annual rings are seen distinctly whereas in hard wood they are indistinct.
2. The colour of soft wood is light whereas the colour of hard wood is dark.
3. Softwoods have lesser strength in compression and shear compared to hardwoods.
4. Softwoods are light and hard woods are heavy.
5. Fire resistance of soft wood is poor compared to that of hardwood.
6. The structure of soft wood is resinous while structure of hard wood is close grained.
1. Pith:
It is the innermost part of the tree and hence the oldest part of exogeneous tree when the plant becomes old, the pith dies and becomes fibrous and dark. It varies in size and shape.
2. Heart Wood:
This is the portion surrounding pith. It is dark in colour and strong. This portion
is useful for various engineering purpose. This is the dead part of wood. It consists of several annular rings.
3. SapWood:
It is the layer next to heart wood. It denotes recent growth and contains sap. It
takes active part in the growth of trees by allowing sap to move in upward direction. The annual rings of sapwood are less sharply divided and are light in colour. The sapwood is also known as alburnum.
4. Cambium Layer:
It is a thin layer of fresh sap lying between sapwood and the inner bark. It
contains sap which is not yet converted into sapwood. If the bark is removed and cambium layer is exposed to atmosphere, cells cease to be active and tree dies.
5. Inner Bark:
It is a inner skin of tree protecting the cambium layer. It gives protection to
cambium layer.
6. Outer Bark:
It is the outer skin of the tree and consists of wood fibres. Sometimes it contains
fissures and cracks.
7. Medullary Rags:
These are thin radial fibres extending from pith to cambium layer. They hold annular rings together. In some of trees they are broken and some other they may not be prominent.
(b) Endogeneous Trees:
These trees grow inwards. Fresh fibrous mass is in the innermost portion. Examples of endogenous trees are bamboo and cane. They are not useful for structural works.
(ii) Classification Based on Modulus of Elasticity:
Young’s modulus is determined by conducting bending test. On this basis timber is classified as:
Group A: E = 12.5 kN/mm2
Group B: E = 9.8 kN/mm2 to 12.5 kN/mm2
Group C: E = 5.6 kN/mm2 to 9.8 kN/mm2.
(iii) Classification Based on Durability:
Durability tests are conducted by the forest research establishment. They bury test specimen of size 600 × 50 × 50 mm in the ground to half their length and observe their conditions regularly over several years. Then timbers are classified as:
High durability:- If average life is more than 10 years.
Moderate durability:- Average life between 5 to 10 years.
Low durability:- Average life less than 5 years.
(iv) Classification Based on Grading:
IS 883-1970 classifies the structural timber into three grades-select grade, grade I and grade II. The classification is based on permissible stresses, defects etc.
(v) Classification Based on Availability: Forest departments classify timbers based on the availability as
X—Most common. 1415 m3 or more per year
Y—Common. 355 m3 to 1415 m3 per year
Z—Less common. Less than 355 m3 per year.
Properties of Timber
Properties of good timbers are:
Colour: It should be uniform.
Odour: It should be pleasant when cut freshly.
Soundness: A clear ringing sound when struck indicates the timber is good.
Texture: Texture of good timber is fine and even.
Grains: In good timber grains are close.
Density: Higher the density stronger is the timber.
Hardness: Harder timbers are strong and durable.
Warping: Good timber do not warp under changing environmental conditions.
Toughness: Timber should be capable of resisting shock loads.
Abrasion: Good timber do not deteriorate due to wear. This property should be looked into, if timber is to be used for flooring.
Strength: Timber should have high strength in bending, shear and direct compression.
Modulus of Elasticity: Timber with higher modulus of elasticity are preferred in construction.
Fire resistance: A good timber should have high resistance to fire.
Permeability: Good timber has low water permeability.
Workability: Timber should be easily workable. It should not clog the saw.
Durability: Good timber is one which is capable of resisting the action of fungi and insects attack
Defects: Good timber is free from defects like dead knots, shakes and cracks.
Seasoning of Timber
This is a process by which moisture content in a freshly cut tree is reduced to a suitable level. By doing so the durability of timber is increased. The various methods of seasoning used may be classified into:
(i) Natural seasoning
(ii) Artificial seasoning.
(i) Natural Seasoning:
It may be air seasoning or water seasoning. Air seasoning is carried outin a shed with a platform. On about 300 mm high platform timber balks are stacked as shown in Care is taken to see that there is proper air circulation around each timber balk. Over a period, in a natural process moisture content reduces. A well seasoned timber contains only 15% moisture.
This is a slow but a good process of seasoning. Water seasoning is carried out on the banks of rivers. The thicker end of the timber is kept pointing upstream side. After a period of 2 to 4 weeks the timber is taken out. During this period sap contained in the timber is washed out to a great extent. Then timber is stalked in a shed with free air circulation.
(ii) Artificial Seasoning:
In this method timber is seasoned in a chamber with regulated heat, controlled humidity and proper air circulation. Seasoning can be completed in 4 to 5 days only. The different methods of seasoning are:
(a) Boiling
(b) Kiln seasoning
(c) Chemical seasoning
(d) Electrical seasoning.
(a) Boiling:
In this method timber is immersed in water and then water is boiled for 3 to 4 hours.Then it is dried slowly. Instead of boiling water hot steam may be circulated on timber. The process of seasoning is fast, but costly.
(b) Kiln Seasoning:
Kiln is an airtight chamber. Timber to be seasoned is placed inside it. Then
fully saturated air with a temperature 35°C to 38°C is forced in the kiln. The heat gradually reaches inside timber. Then relative humidity is gradually reduced and temperature is increased, and maintained till desired degree of moisture content is achieved.
The kiln used may be stationary or progressive. In progressive kiln the carriages carrying timber travel from one end of kiln to other end gradually. The hot air is supplied from the discharging end so that temperature increase is gradual from charging end to discharging end. This method is used for seasoning on a larger scale.
(c) Chemical Seasoning:
In this method, the timber is immersed in a solution of suitable salt.
Then the timber is dried in a kiln. The preliminary treatment by chemical seasoning ensures uniform seasoning of outer and inner parts of timber.
(d) Electrical Seasoning:
In this method high frequency alternate electric current is passed through timber. Resistance to electric current is low when moisture content in timber is high. As moisture content reduces the resistance reduces.
Measure of resistance can be used to stop seasoning at appropriate level.However it is costly process. This technique has been tried in some plywood industries but not in seasoning of timber on mass scale.
Defects in Timber
Various defects which are likely to occur in timber may be grouped into the following three:
(i) Due to natural forces
(ii) Due to defective seasoning and conversions.
(iii) Due to attack by fungi and insects.
(i) Defects due to Natural Forces:
The following defects are caused by natural forces:
(a) Knots
(b) Shakes
(c) Wind cracks
(d) Upsets
(a) Knots:
When a tree grows, many of its branches fall and the stump of these branches in the trunk is covered. In the sawn pieces of timber the stump of fallen branches appear as knots. Knots are dark and hard pieces. Grains are distorted in this portion. Figure 1.9 shows some varieties of knots. If the knot is intact with surrounding wood, it is called live knot. If it is not held firmly it is dead knot.
(b) Shakes:
The shakes are cracks in the timber which appear due to excessive heat, frost or
twisting due to wind during the growth of a tree. Depending upon the shape and the positions shakes can be classified as star shake, cup shake, ring shakes and heart shakes
(c) Wind Cracks:
These are the cracks on the outside of a log due to the shrinkage of the exterior surface.
(d) Upsets:
This type of defect is due to excessive compression in the tree when it was young. Upset is an injury by crushing. This is also known as rupture.
(ii) Defects due to Defective Seasoning and Conversion:
If seasoning is not uniform, the converted timber may warp and twist in various directions. Sometimes honey combining and even cracks appear. This type of defects are more susceptible in case of kiln seasoning.In the process of converting timber to commercial sizes and shapes the following types of defects are likely to airse: chip marks, torn grain etc.
(iii) Defects due to Fungi and Insects Attack:
Fungi are minute microscopic plant organism.They grow in wood if moisture content is more than 20°C and exposed to air. Due to fungi attack rotting of wood, takes place. Wood becomes weak and stains appear on it.Beetles, marine borers and termites (white ants) are the insects which eat wood and weaken the timber. Some woods like teak have chemicals in their compositions and resist such attacks. Other woods are to be protected by chemical treatment.
Preservation of Timber
Preservation of timber means protecting timber from fungi and insects attack so that its life is increased. Timber is to be seasoned well before application of preservatives. The following are the widely used preservatives:
1. Tar
2. Paints
3. Chemical salt
4. Creosote
5. ASCO
1. Tar:
Hot coal tar is applied to timber with brush. The coating of tar protects the timber from the attack of fungi and insects. It is a cheapest way of protecting timber. Main disadvantage of this method of preservation is that appearance is not good after tar is applied it is not possible to apply other attractive paints. Hence tarring is made only for the unimportant structures like fence poles.
2. Paints:
Two to three coats of oil paints are applied on clean surface of wood. The paint
protects the timber from moisture. The paint is to be applied from time to time. Paint improves the appearance of the timber. Solignum paint is a special paint which protects the timber from the attack of
termites.
3. Chemical salt:
These are the preservatives made by dissolving salts in water. The salts used
are copper sulphate, masonry chloride, zinc chloride and sodium fluoride. After treating the timber with these chemical salt paints and varnishes can be applied to get good appearance.
4. Creosote:
Creosote oil is obtained by distillation of coal tar. The seasoned timber is kept in
an air tight chamber and air is exhausted. Then creosote oil is pumped into the chamber at a pressure of 0.8 to 1.0 N/mm2 at a temperature of 50°C. After 1 to 2 hours timber is taken out of the chamber.
5. ASCO:
This preservative is developed by the Forest Research Institute, Dehradun. It consists of 1 part by weight of hydrated arsenic pentoxide (As2O5, 2 H2O), 3 parts by weight of copper sulphate (CuSO4⋅5 H2O) and 4 parts by weight of potassium dichromate (K2Cr2O7) or sodium dichromate (Na2Cr2O7⋅2 H2O). This preservative is available in powder form. By mixing six parts of this powder with 100 parts of water, the solution is prepared. The solution is then sprayed over the surface of timber. This treatment prevents attack from termites. The surface may be painted to get desired appearance.
